Fiber-coupled scintillation detectors of radiation and particles have been employed over the course of the past 30 years. In some cases, the scintillator is pixelated, consisting of discrete scintillator elements, and in other cases, other stratagems are employed (such as orthogonally crossed coupling fibers) in order to provide spatial resolution. Examples of fiber-coupled scintillation detectors are provided by U.S. Pat. No. 6,078,052 (to DiFilippo) and U.S. Pat. No. 7,326,9933 (to Katagiri et al.), both of which are incorporated herein by reference. Detectors described both by DiFilippo and Katagiri et al. employ wavelength-shifting fibers (WSF) such that light reemitted by the core material of the fiber may be conducted, with low attenuation, to photo-detectors disposed at a convenient location, often distant from the scintillator itself. Spatial resolution is of particular value in applications such as neutron imaging. Spatial resolution is also paramount in the Fermi Large Area Space Telescope (formerly, GLAST) where a high-efficiency segmented scintillation detector employs WSF readout for detection of high-energy cosmic rays, as described in Moiseev, et al., High efficiency plastic scintillator detector with wavelength-shifting fiber readout for the GLAST Large Area Telescope, Nucl. Instr. Meth. Phys. Res. A, vol. 583, pp. 372-81 (2007), which is incorporated herein by reference.
Because of the contexts in which fiber-coupled scintillator detectors have been employed to date, all known fiber-coupled scintillator detectors have counted pulses produced by individual interactions of particles (photons or massive particles) with the scintillator, thereby allowing the energy deposited by the incident particle to be ascertained based on the cumulative flux of light reemitted by the scintillator.
The detection requirements of X-ray backscatter inspection systems, however, are entirely different from the requirements addressed by existing fiber-coupled scintillation detectors. Backscatter X-ray inspection systems have been used for more than 25 years to detect organic materials concealed inside baggage, cargo containers, in vehicles, and on personnel. Because organic materials in bulk preferentially scatter X rays (by Compton scattering) rather than absorb them, these materials appear as brighter objects in backscatter images. Insofar as incident X-rays are scattered into all directions, sensitivity far overrides spatial resolution as a requirement, and in most scatter applications, detector spatial resolution is of no concern at all, since resolution is governed by the incident beam rather than by detection.
The specialized detection requirements of large area and high sensitivity posed by X-ray scatter systems are particularly vexing in the case of “conventional” scintillation detectors 100 of the type shown in a side cross-section in FIG. 1A and in a front cross-section in FIG. 1B. An example of such a detector is described in U.S. Pat. No. 5,302,817 (to Yokota), and is incorporated herein by reference. Typically, a light-tight box 102 is lined with scintillating screens 103 where incident X-ray radiation 101 is converted to scintillation light, typically in the UV, visible, or longer wavelength, portions of the electromagnetic (EM) spectrum. Large-photocathode-area photomultiplier tubes (PMTs) 105 are coupled to receive scintillation light via portholes 108. One problem lies in that a fraction of the scintillation light originating within the screen is transmitted from the screen into the enclosed volume. The remaining scintillation light is lost in the screen material. Scintillating screens 103 are designed to maximize the fraction of emitted light, which is tantamount to ensuring a large transmission coefficient T for the interface between screen 103 and the medium (typically air) filling the detector volume. However, in a conventional backscatter detector of the sort depicted in FIGS. 1A and 1B, the scintillation screens 103 should also serve as good reflectors because scintillation light, once emitted into the volume of box 102, typically needs multiple reflections until it reaches a photo-detector 105. So, the reflection coefficient R of the screen surface should also be large; however, since the sum of T and R is constrained to be unity, both T and R cannot be maximized simultaneously, and a compromise must be struck. As a result, the light collection efficiency of the conventional backscatter detector is inherently low, with only a few percent of the generated scintillation light collected into the photo detectors.
For an imaging detector, the photon statistical noise is calculated in terms of the photons absorbed by the detector and used to generate the image. Any photons which pass through the detector without being absorbed, or even those that are absorbed without generating image information, are wasted and do not contribute to reducing noise in the image. Since photons cannot be subdivided, they represent the fundamental quantum level of a system. It is common practice to calculate the statistical noise in terms of the smallest number of quanta used to represent the image anywhere along the imaging chain. The point along the imaging chain where the fewest number of quanta are used to represent the image is called a “quantum sink”. The noise level at the quantum sink determines the noise limit of the imaging system. Without increasing the number of information carriers (i.e., quanta) at the quantum sink, the system noise limit cannot be improved. Poor light collection can possibly create a secondary quantum sink, which is to say that it will limit the fraction of incident X rays resulting in PMT current. Moreover, it will increase image noise. Light collection efficiency can be improved by increasing the sensitive area of the photo-detectors, however, that path to efficiency is costly.
The structure of a scintillating screen typically employed in prior art X-ray scintillation detectors is now described with reference to FIG. 2. A layer of composite scintillator 202 is sandwiched between a backer sheet 204 for structural support and a thin, transparent protective film 206 composed of polyester, for example. The composite scintillator consists of typically micron-size inorganic crystals in an organic matrix or resin. The crystals are the actual scintillating material. Barium fluoro-chloride (BaFCl, or “BFC”) or gadolinium oxysulfide (Gd2O2S, or “Gadox”) doped with rare earth elements are common choices for these. The stopping power of the screen is determined by the thickness of the composite scintillator layer 202, which is typically measured in milligrams of scintillator crystal per unit area. Because the inorganic scintillators (such as BFC or Gadox) suffer from high self-absorption, the composite scintillator layer has to be kept rather thin in order to extract a good fraction of the scintillation light. This limits the useful stopping power of the screen and makes it suitable only for the detection of X rays with energies up to around 100 keV. It would be advantageous to have scintillation detectors for X-ray scatter detection applications that provide for more efficient extraction, collection, and detection of scintillation light.
Scintillator structures have been produced using many manufacturing technologies, including, for example, die-casting, injection molding (as described by Yoshimura et al., Plastic scintillator produced by the injection-molding technique, Nucl. Instr. Meth. Phys. Res. A, vol. 406, pp. 435-41 (1998), and extrusion, (as described in U.S. Pat. No. 7,067,079, to Bross, et al.), both of which references are incorporated herein by reference.
As briefly discussed above, wavelength-shifting fibers (WSF) have long been employed for scintillation detection. Wavelength shifting fibers consist of a core with relatively high refractive index, surrounded by one or more cladding layers of lower refractive index. The core contains wavelength-shifting material, also referred to as dye. Scintillation light which enters the fiber is absorbed by the dye which, in turn, emits light with a longer wavelength. The longer wavelength light is emitted isotropically in the fiber material. Total internal reflection traps a fraction of that light and conducts it over long distances with relatively low loss. This is possible, as described with reference to FIG. 3A, because the absorption 304 and emission 302 wavelength ranges of the dye effectively do not overlap so that the wavelength-shifted light is not reabsorbed. The captured fraction is determined by the ratio of the refractive indices at surfaces of the fiber. An additional advantage of WSF is that the wavelength shifting can bring the scintillation light 306 into the sensitive wavelength range of the photo detector (PMT, silicon photomultiplier (SiPM), or Multiple-Pixel Photon-Counter (MPPC), or otherwise).
The use of WSF detectors in a flying spot X ray imaging system is known. A flying-spot scanner (FSS) uses a scanning source that is a spot of light, such as but not limited to, a high-resolution, high-light-output, low-persistence cathode ray tube (CRT), to scan an image. In contrast with film or digital X-ray detectors which have spatially sensitive detectors that establish the system resolution, flying spot X-ray systems are limited by the illumination beam spot size. The illumination beam spot size is determined by a number of factors including the X-ray focal spot size, the collimation length, the aperture size and the distance to the target.
The beam spot is the pinhole image of the focal spot, geometrically blurred by the relatively large size of the pinhole or aperture. In general, the shape of the aperture is substantially similar to the shape of the focal spot but typically larger. Accordingly, any internal structure is blurred out and only the overall dimension of the focal spot is relevant. The ideal beam spot is a sharp disk or rectangle. In reality, however, the edges are blurred. The umbra region is obtained by the projection of the aperture from the equivalent/virtual point source, which is, however, only well-defined for the case of round disk-shaped focal spot. The umbra region is defined as the region in which the entirety of the light source is obscured by the occluding body being imaged; while the penumbra is the image region where in which only a portion of the light source is obscured by the occluding body.
The actual two-dimensional intensity distribution describing the beam spot is controlled by the combination of both the focal spot and the collimator. Known collimation designs strive to minimize the size of the penumbra, as shown in FIG. 3B, which illustrates the relationship between focal length and umbra and penumbra diameters with the light and dark regions reversed. As shown, a width 310 of the penumbra correlates to and scales with the ratio of a focal spot 312 size to collimation length 314, which is the distance between focal spot 312 and aperture 316, as defined by the source 318 and the target 320. Consequently, a small focal spot allows for a shorter collimator (more compact and lighter design) and/or better collimation (smaller penumbra).
Mathematically, an umbra diameter (UD) 320 and a penumbra width (PW) 310 are related to the diameter of focal spot 312 (FS), the diameter of the aperture (AD) 322, collimation length (CL) 314, and target distance (TD) 324 through the following equations:UD=AD+TD/CL(AD−FS)  (1)PW=FS*TD/CL  (2)
The resolution of currently available flying spot X-ray imaging systems is limited by the size of the flying spot. The detection system has little to no spatial sensitivity, and as a result, the spatial information is created by moving the spot across the detector with synchronization to the detector readout over time. The minimum spot size is limited by the X-ray source spot size and the collimation system used to generate the spot. Typically, in cargo imaging systems, the spot is 7-10 mm in size at the detector. As shown by the equations (1) and (2) above, reducing the size of the focal spot enables designing short length collimators for flying spot X-ray imaging systems and obtaining sharper images.
In light of the above, there is clearly a need for increased spatial sensitivity for X-ray detectors in a flying spot imaging system. There is also a need to develop a WSF system that is capable of determining both the high resolution content of an image as well as the low resolution mapping of the coarse location of the spot. Furthermore, there is a need to be able to generate a high resolution image with a minimum of individual channels, thus saving cost and complexity of the system. Finally, there is a need for an improved detection system that could be effectively used in any flying spot x-ray system and configured to generate improved resolution in one or two dimensions.